MDC propagation

Following up on discussion here, it seems the root issue is that the MDC is lost when a play-ws result is mapped. I’m not sure how this solution worked before, but I think a possible fix would be ability to configure the ExecutorService/ExecutionContext associated with play-ws.

I created an example solution using sttp/okhttp, which allows me to do that.

Curious what people think of this solution, and if it would be possible to apply similar technique to play-ws/AHC.